1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing device that fixes a toner image onto a recording material.
2. Description of the Related Art
In the related art, an image forming apparatus, such as a copying machine or a printer employing an electrophotographic system, is provided with a fixing device employing a heat-fixing system as a unit that performs a fixing process on a recording material to which a toner image has been transferred. The fixing device includes a fixing film that rotates along an arc-like film guide, a heat source that is disposed at an inner side of the fixing film and that heats a recording material via the fixing film, and a pressure roller in which a heat-resistance elastic layer, such as a rubber material fixed to a mandrel, is coated with a resin tube.
In the fixing device, the film guide that supports the fixing film so as to be rotatable is impelled to the pressure roller by an elastic member, such as a coil spring, to press the fixing film against the pressure roller, and unfixed toner is fixed to a recording material on which the unfixed toner has been transferred by passing the recording material through a fixing nip portion formed by the pressurization to heat and pressurize the recording material.
In a dry environment in which humidity is low, when a recording material is passed to the fixing device, the surface layer of the pressure roller is gradually charged with a negative polarity by frictional charging between the transported recording material and the pressure roller and an electrostatic offset occurs in which negatively-charged toner on the recording material is attached to the fixing film. The electrostatic offset is prevented by applying a negative bias, which is higher than the negative voltage of the pressure roller, to the fixing film or replacing the rubber material of the pressure roller with a conductive material to lower the internal resistance of the pressure roller in order to suppress the electrostatic offset based on the negative charging of the pressure roller. However, with an increase in the a image forming speed, there is a problem in that the negative charging amount of the pressure roller increases due to an increase in a frictional charging amount and an increase in the applied negative bias.
Therefore, for example, Japanese Patent Laid-Open No. 2010-128474 discloses a configuration in which a high bias having the opposite-polarity of a fixing bias is applied in a period in which a recording material does not pass through the fixing nip portion. By applying the bias having the opposite-polarity in this way, an electric field is generated in the fixing nip portion to remove electricity of an insulating layer, which is the surface layer of the pressure roller, charged with the negative polarity in the period in which a recording material does not pass therethrough, whereby image failure, such as the electrostatic offset which occurs by charging the insulating layer, is prevented.
Here, when foreign substances such as a staple along with a recording material is transported to the fixing device, a hole may be formed in a film-like resin tube constituting the surface layer of the pressure roller in the fixing nip portion. When the hole has a small diameter, the hole does not affect the fixing operation and the fixing device can be continuously used. However, when a high bias having the opposite-polarity is applied to remove electricity from the surface layer of the pressure roller, even a hole having a small diameter may cause a problem with the application of the high bias. When a hole is formed in the insulating layer as the surface layer (resin tube) of the pressure roller, a conductive rubber material, which is grounded via the mandrel, is exposed from the hole. When a high bias having the opposite-polarity to the charging polarity of the pressure roller is applied to the fixing film to remove electricity, electric discharge occurs between the hole and the fixing film due to a potential difference between the grounded conductive rubber material of the pressure roller in the hole and the fixing film when the distance between the hole and the fixing film reaches a predetermined minute distance. In the place in which the electric discharge occurs, a coating layer, which is the surface layer of the fixing film, is damaged by the electric discharge and toner parting properties of the coating layer deteriorate. When the toner parting properties deteriorate, toner is attached to the surface of the fixing film and the attached toner is attached to a recording material, which is transported from the fixing nip portion, thereby causing image contamination.